A number of electrophysiological studies have been devoted to the functional organization of trigeminal nucleus oralis (TNO) with only a very few anatomical analyses directed toward an understanding of the structural substrates underlying the electrophysiological events. TNO has been implicated in the reception, central processing and modification and the transmission of neural information related to oral-facial nociceptive (pain) and mechanoreceptive (tactile) stimuli and it is perceived as being involved in numerous chronic pain states such as trigeminal neuralgia (tic douloureux). TNO is a major target of primary trigeminal neurons which innervate the tooth pulp where it is generally considered that pain is the only sensation that can be elicited in man. The overall objectives of my studies are to delineate the synaptic circuitry in TNO in order to understand and explain how nociceptive and mechanoreceptive information transmitted from the oral-facial region by primary trigeminal neurons is processed before being relayed to other centers in the brain. This proposed project is concerned with the synaptic connections of the sensory root of the trigeminal nerve and the morphology of the neurons which comprise TNO. These studies employ light (LM) and electron (EM) microscopy, the Golgi method and the intra-axonal anterograde horseradish peroxidase (HRP) labeling technique. The specific aims of the proposed research are: (1) to demonstrate the distribution of primary trigeminal afferent axons to TNO of the adult rat; (2) to examine the morphological features of individual HRP-filled terminal axonal arbors of primary trigeminal afferent axons in TNO with LM and then evaluate their cytology and synaptic relations with EM; and (3) to identify the neuronal cell types in TNO and assess the overall morphology and distribution of their axons and dendrites. The knowledge gained from these studies will provide an essential anatomical framework for understanding the synaptic circuitries through which TNO processes oral-facial nociceptive and mechanoreceptive information under normal and pathological circumstances. Such an understanding, in light of concurrent pharmacological advances in drug therapy, should contribute to the successful treatment of trigeminal related chronic pain states which affect the face and oral cavity.